The present invention relates generally to the treatment of viral infections using lipid derivatives of antiviral nucleoside analogues. More particularly, the present invention relates to lipid, and especially phospholipid, derivatives of modified antiviral nucleoside analogues which can be integrated into the structure of liposomes, thereby forming a more stable liposomal complex which can deliver greater amounts of drugs to target cells with less toxicity.
The publications and other reference materials referred to herein are hereby incorporated by reference, and are listed for convenience in the bibliography appended at the end of this specification.
There has been a great deal of interest in recent years in the use of nucleoside analogues to treat viral infections. A nucleoside consists of a pyrimidine or purine base which is linked to ribose, a five-carbon sugar having a cyclic structure. The antiviral nucleoside analogues closely resemble natural nucleosides and are designed to inhibit viral functions by preventing the synthesis of new DNA or RNA. Nucleosides are enzymatically assembled into DNA or RNA.
During DNA synthesis, free nucleoside triphosphates (nucleosides with three phosphate groups attached) react with the end of a growing DNA chain. The reaction involves the linking of the phosphate group at the 5' position on the incoming nucleoside triphosphate with the hydroxyl group at the 3' position of the sugar ring on the end of the forming DNA chain. The other two phosphate groups are freed during the reaction, thereby resulting in the addition of a nucleotide to the DNA chain.
Nucleoside analogues are compounds which mimic the naturally occurring nucleosides sufficiently so that they are able to participate in viral DNA synthesis. However, the antiviral nucleoside analogues have strategically located differences in chemical structure which inhibit viral enzymes such as reverse transcriptase or which prevent further DNA synthesis once the analogue has been attached to the growing DNA chain.
Dideoxynucleosides are antiviral compounds that lack the hydroxyl groups normally present at the second and third position of ribose. When a dideoxynucleoside is incorporated into a growing DNA chain, the absence of the 3--OH group on its ribose group makes it impossible to attach another nucleotide and the chain is terminated. Dideoxynucleosides are particularly useful in treating retroviral infections where viral replication requires the transcription of viral RNA into DNA by viral reverse transcriptase. Other nucleoside analogues include deoxynucleosides and nucleosides analogues having only a fragment of ribose or other pentose connected to the base molecule.
Acquired immunodeficiency syndrome (AIDS) is caused by the human immunodeficiency virus (HIV). HIV infects cells bearing the CD4 (T4) surface antigen, such as CD4+ helper lymphocytes, CD4+ monocytes and macrophages and certain other CD4+ cell types. The HIV infection of CD4+ lymphocytes results in cytolysis and cell death which contributes to the immunodeficiency of AIDS; however, CD4+ monocytes and macrophages may not be greatly harmed by the virus. Viral replication in these cells appears to be more prolonged and less cytotoxic than in lymphocytes, and as a result, monocytes and macrophages represent important reservoirs of HIV infection. It has recently been discovered that macrophages may serve as reservoirs of HIV infection even in certain AIDS patients who test negative for the presence of HIV antibodies. No effective cure is available for AIDS, although dideoxynucleosides have been shown to prolong life and to reduce the incidence of certain fatal infections associated with AIDS.
Certain monocyte-derived macrophages, when infected with some strains of HIV, have been found to be resistant to treatment with dideoxycytidine, azidothymidine, and other dideoxynucleosides in vitro as shown by Richman, et al. (1). The resistance may be due in part to the low levels of dideoxynucleoside kinase which result in a reduced ability to phosphorylate AZT, ddC or ddA. Clearly, it would be useful to have more effective ways of delivering large amounts of effective antiviral compounds to macrophages infected with HIV or other viruses and other cells having viral infections. It would also be useful to have more effective ways of delivering antiviral compounds which not only increase their potency but prolong their efficacy.
Dideoxynucleoside analogues such as AZT are the most potent agents currently known for treating AIDS, but in a recent human trial, serious toxicity was noted, evidenced by anemia (24%) and granulocytopenia (16%) (2,3). It is desirable, therefore, to provide a means for administering AZT and other dideoxynucleosides in a manner such that the toxic side effects of these drugs are reduced. Further, it is desirable to provide selective targeting of the dideoxynucleoside to monocyte/macrophages to enhance the efficiency of the drug against viral infection in this group of cells. One way to do this is to take advantage of the uptake of liposomes by macrophages.
In 1965, Alex Bangham and coworkers discovered that dried films of phosphatidylcholine spontaneously formed closed bimolecular leaflet vesicles upon hydration (4). Eventually, these structures came to be known as liposomes.
A number of uses for liposomes have been proposed in medicine. Some of these uses are as carriers to deliver therapeutic agents to target organs. The agents are encapsulated during the process of liposome formation and released in vivo when liposomes fuse with the lipids of cell surface membrane. Liposomes provide a means of delivering higher concentrations of therapeutic agents to target organs. Further, since liposomal delivery focuses therapy at the site of liposome uptake, it reduces toxic side effects.
For example, liposomal antimonial drugs are several hundred-fold more effective than the free drug in treating leishmaniasis as shown independently by Black and Watson (5) and Alving, et al. (6). Liposome-entrapped amphotericin B appears to be more effective than the free drug in treating immunosuppressed patients with systemic fungal disease (7). Other uses for liposome encapsulation include restriction of doxorubicin toxicity (8) and diminution of aminoglycoside toxicity (9).
As previously mentioned, it is now thought that macrophages are an important reservoir of HIV infection (10, 11). Macrophages are also a primary site of liposome uptake (12, 13). Accordingly, it would be desirable to utilize liposomes to enhance the effectiveness of antiviral nucleoside analogues in treating AIDS and other viral infections.
The use of liposomes to deliver phosphorylated dideoxynucleoside to AIDS infected cells which have become resistant to therapy has been proposed in order to bypass the low dideoxynucleoside kinase levels.
Attempts have also been made to incorporate nucleoside analogues, such as iododeoxyuridine (IUDR), acylovir (ACV) and ribavirin into liposomes for treating diseases other than AIDS. However, these attempts have not been entirely satisfactory because these relatively small water soluble nucleoside analogues tend to leak out of the liposome rapidly (14, 15), resulting in decreased targeting effectiveness. Other disadvantages include the tendency to leak out of liposomes in the presence of serum, difficulties in liposome formulation and stability, low degree of liposomal loading, and hydrolysis of liposomal dideoxynucleoside phosphates when exposed to acid hydrolases after cellular uptake of the liposomes.
Attempts have also been made to combine nucleoside analogues, such as arabinofuranosylcytosine (ara-C) and arabinofuranosyladenine (ara-A), with phospholipids in order to enhance their catabolic stability as chemotherapeutic agents in the treatment of various types of cancer (16). The resulting agents showed a decreased toxicity and increased stability over the unincorporated nucleoside analogues. However, the resulting agents exhibited poor cellular uptake (16) and poor drug absorption (17).
In order to use nucleoside analogues incorporated into liposomes for treating viral infections more effectively, it is desirable to increase the stability of the association between the liposome and the nucleoside analogue.
In order to further enhance the effectiveness of these antiviral liposomes, it would be desirable to target the liposomes to infected cells or sites of infection. Greater specificity in liposomal delivery may be obtained by incorporating monoclonal antibodies or other ligands into the liposomes. Such ligands will target the liposomes to sites of liposome uptake capable of binding the ligands. Two different approaches for incorporating antibodies into liposomes to create immunoliposomes have been described: that of Huang and coworkers (18) involving the synthesis of palmitoyl antibody, and that of Leserman, et al. (19) involving the linkage of thiolated antibody to liposome-incorporated phosphatidylethanolamine (PE).
The methods disclosed here apply not only to dideoxynucleosides used in the treatment of AIDS and other retroviral diseases, but also to the use of antiviral nucleosides in the treatment of diseases caused by other viruses, such as herpes simplex virus (HSV), human herpes virus 6, cytomegalovirus (CMV), hepatitis B virus, Epstein-Barr virus (EBV), and varicella zoster virus (VZV). Thus, the term "nucleoside analogues" is used herein to refer to compounds that can inhibit viral replication at various steps, including inhibition of viral reverse transcriptase or which can be incorporated into viral DNA or RNA, where they exhibit a chain-terminating function.